1. Field of the Invention
The present invention relates to a display device and a method of fabricating a display device, and more particularly, to an active matrix organic electroluminescent display device and method of fabricating an active matrix organic electroluminescent display device.
2. Discussion of the Related Art
An organic electroluminescent display device includes a cathode electrode to inject electrons, an anode electrode inject to holes, and an organic electroluminescent layer between the two electrodes. An organic electroluminescent diode has a multi-layer structure of organic thin films provided between the anode electrode and the cathode electrode. When a forward current is applied to the organic electroluminescent diode, electron-hole pairs (often referred to as excitons) combine in the organic electroluminescent layer as a result of a P-N junction between the anode electrode, which injects holes, and the cathode electrode, which injects electrons. The electron-hole pairs have a lower energy when combined than when they were separated. The resultant energy gap between the combined and separated electron-hole pairs is converted into light by an organic electroluminescent element. In other words, the organic electroluminescent layer emits the energy generated due to the recombination of electrons and holes in response to an applied current.
As a result of the above-described principles, the organic electroluminescent device does not need an additional light source as compared with a liquid crystal display device. Moreover, the electroluminescent device is thin, light weight, and is very energy efficient. As a result, the organic electroluminescent device has excellent advantages when displaying images, such as a low power consumption, high brightness, and a short response time. Because of these advantageous characteristics, the organic electroluminescent device is regarded as a promising candidate for various next-generation consumer electronic appliances, such as mobile communication devices, CNS (car navigation system), PDAs (personal digital assistances), camcorders, and palm PCs. Also, because the fabricating of such organic electroluminescent devices is a relatively simple process, an organic electroluminescent device is cheaper to produce than a liquid crystal display device.
Organic electroluminescent display devices may be provided in either a passive matrix type arrangement or an active matrix type arrangement. The passive matrix type has a simple structure and fabrication process, but has a high power consumption as compared to the active matrix type. Further, because the display size of passive matrix organic electroluminescent display devices is limited by its structure, the passive matrix type can not easily be adapted to a large sized device. Moreover, the aperture ratio of the passive matrix type decreases as the bus lines increases. In contrast, active matrix type organic electroluminescent display devices provide a higher display quality with higher luminosity as compared to the passive matrix type.
FIG. 1 is a schematic cross-sectional view illustrating an active matrix type organic electroluminescent display device according to a related art arrangement. As shown in FIG. 1, an organic electroluminescent display device 10 includes first and second substrates 12 and 28, which are attached to each other by a sealant 26. On the first substrate 12, a plurality of thin film transistors (TFTs) T and array portions 14 are formed. Each of the TFTs T corresponds to each pixel region P. A first electrode (i.e., an anode electrode) 16, an organic luminous layer 18 and a second electrode (i.e., a cathode electrode) 20 are sequentially formed on the array portion 14. At this point, the organic luminous layer 18 emits red (R), green (G) or blue (B) color in each pixel P. In particular, to show color images, organic color luminous patterns are disposed respectively in each pixel P.
As additionally shown in FIG. 1, the second substrate 28, which is attached to the first substrate 12 by the sealant 26, includes a moisture absorbent 22 on the rear surface thereof. The moisture absorbent 22 absorbs the moisture that may exist in the cell gap between the first and second substrates 12 and 28. When disposing the moisture absorbent 22 in the second substrate 28, a portion of the second substrate 28 is etched to form a dent. Thereafter, a powder-type moisture absorbent 22 is disposed into this dent, and then, a sealing tape 25 is put on the second substrate 28 to fix the powder-type moisture absorbent 22 into the dent.
FIG. 2 is an equivalent circuit diagram illustrating a pixel of the organic electroluminescent display device according to a related art arrangement. As shown in FIG. 2, a gate line GL is disposed in a transverse direction and a data line DL is disposed in a longitudinal direction substantially perpendicular to the gate line GL. A switching thin film transistor (switching TFT) TS is disposed in a crossing of the gate and data lines GL and DL and a driving thin film transistor (driving TFT) TD is disposed electrically connecting with the switching thin film transistor TS. The driving TFT TD is electrically connected with an organic electroluminescent diode E. A storage capacitor CST is disposed between a power line PL and a drain S6 of the switching TFT TS. The storage capacitor CST is also connected to a gate D2 of the driving TFT TD. A source S4 of the switching TFT TS is connected to the data line DL, and a source D4 of the driving TFT TD is connected to the power line PL. The organic electroluminescent diode E comprises a first electrode, an organic luminous layer and a second electrode, as described in FIG. 1. The first electrode of the organic electroluminescent diode E electrically contacts with a drain D6 of the driving TFT TD, the organic luminous layer is disposed on the first electrode, and the second electrode is disposed on the organic luminous layer.
Now, an operation of the organic electroluminescent display device will be briefly explained with reference to FIG. 2. When a gate signal is applied to a gate S2 of the switching TFT TS from the gate line GL, a data current signal flowing via the data line DL is converted into a voltage signal by the switching TFT TS to be applied to the gate D2 of the driving TFT TD. Thereafter, the driving TFT TD is operated and determines a current level that flows through the organic electroluminescent diode E. As a result, the organic electroluminescent diode E can display a gray scale between black and white.
The voltage signal is also applied to the storage capacitor CST such that a charge is stored in the storage capacitor CST. The charge stored in the storage capacitor CST maintains the voltage of the voltage signal on the gate S2 of the driving TFT TD. Thus, although the switching TFT TS is turned off, the current level flowing to the organic electroluminescent diode E remains constant until the next voltage signal is applied.
Meanwhile, the switching and driving TFTs TS and TD may include either of a polycrystalline silicon layer or an amorphous silicon layer. When the TFTs TS and TD include an amorphous silicon layer, fabrication of the TFTs TS and TD is more simple as compared to TFTs TS and TD that include a polycrystalline silicon layer.
FIG. 3 is a schematic plan view of an active matrix organic electroluminescent display device having a bottom emission type according to the related art. As shown in FIG. 3, the active matrix organic light emitting diode device includes, for example, inverted staggered type thin film transistors.
A gate line 36 crosses a data line 49 and a power line 62, which are spaced apart from each other. A pixel region is defined between the gate line 36 and the spaced apart data and power supply lines 49 and 62. A switching thin film transistor (TFT) TS is disposed adjacent to where the gate line 36 and the data line 49 cross each other. A driving thin film transistor (TFT) TD is disposed next to the power line 62 and in the pixel region. The driving TFT TD has a larger size than the switching TFT TS, and therefore, the driving TFT TD occupies a relatively large space of the pixel region.
The switching TFT TS includes a switching gate electrode 32 extending from the gate line 36, a switching source electrode 48 extending from the data line 49, a switching drain electrode 50 spaced apart from the switching source electrode 48, and a switching active layer 56a above the switching gate electrode 32. The switching active layer 56a is formed of amorphous silicon and has an island shape.
The driving TFT TD is connected to the switching TFT TS and the power line 62. The driving TFT TD includes a driving gate electrode 34, a driving source electrode 52, a driving drain electrode 54 and a driving active layer 58a. The driving gate electrode 34 is connected with the switching drain electrode 50 and elongates along side the power line 62. The driving active layer 58a is formed of amorphous silicon and has a long island shape. Additionally, the driving active layer 58a also elongates along side the power line 62 while also overlapping the driving gate electrode 34. The driving source and drain electrodes 52 and 54 overlap side portions of the driving gate electrode 34. The driving active layer 58a having an island shape is disposed above the driving gate electrode 34 between the driving source and drain electrodes 52 and 54.
As also shown in FIG. 3, the power line 62 has a protrusion extending to the driving source electrode 50 and electrically communicates with the driving source electrode 50 through the protrusion. A first electrode 66 of the organic electroluminescent diode is disposed in the pixel region and connected with the driving drain electrode 54.
The driving thin film transistor TD needs to have an ability to operate and drive the organic electroluminescent diode. Thus, a channel of the driving thin film transistor TD should have a large channel width W and a short channel length L such that the ratio of width W and length L should be large enough. Thus, the driving thin film transistor TD can supply sufficient current to the organic electroluminescent diode to operate and driving the organic electroluminescent diode.
FIGS. 4 and 5 are cross sectional views taken along lines IV-IV and V-V of FIG. 3 illustrating the switching thin film transistor and the driving thin film transistor, respectively.
In FIGS. 4 and 5, the switching gate electrode 32 and the driving gate electrode 34 is formed on a substrate 30. Although not shown in FIGS. 4 and 5, but shown in FIG. 3, the gate line 36 is also formed on the substrate 30. As described before, the driving gate electrode 34 is larger than the switching gate electrode 32 and occupies a large portion of the pixel region. A gate insulating layer 38 is formed on the substrate to cover the driving and switching gate electrodes 32 and 34 and the gate line 36. The gate insulating layer 38 has a contact hole that exposes a bottom end of the driving gate electrode 34. A switching semiconductor layer 56 and a driving semiconductor layer 58 are formed on the gate insulating layer 38, respectively, above the switching gate electrode 32 and above the driving gate electrode 34. The switching semiconductor layer 56 comprises a switching active layer 56a of pure amorphous silicon and a switching ohmic contact layer 56b of doped amorphous silicon. The driving semiconductor layer 58 is comprises a driving active layer 58a of pure amorphous silicon and a driving ohmic contact layer 58b of doped amorphous silicon. As shown in FIG. 3, the driving semiconductor layer 58 is larger than the switching semiconductor layer 56. The switching source and drain electrodes 48 an 50 are formed spaced apart from each other and contact the switching ohmic contact layer 56b, and the driving source and drain electrodes 52 and 54 are formed spaced apart from each other in contact with the diving ohmic contact layer 58b. The switching drain electrode 50 also electrically contacts the driving gate electrode 34. The data line 49 is also formed on the gate insulating layer 38 and disposed perpendicularly crossing the gate line 36, as shown in FIGS. 3 and 4. Therefore, the switching thin film transistor TS and the driving thin film transistor TD are complete.
A first passivation layer 60 is formed over an entire of the substrate 30 to cover the switching thin film transistor TS and the driving thin film transistor TD. The first passivation layer 60 has a contact hole that exposes the driving source electrode 52. Then, the power line 62 is formed on the first passivation layer 60 and contacts the driving source electrode 52 through the contact hole, as shown in FIG. 5. The power line 62 is spaced apart from the data line 49 and perpendicularly crosses the gate line 36, as shown in FIG. 3, thereby defining the pixel region with the gate and data lines 36 and 49. A second passivation layer 64 is formed over an entire of the substrate 30 to cover the power line 62. The first and second passivation layers 60 and 64 have a contact hole that exposes a portion of the driving source electrode 54 through the contact hole. The first electrode 66 of the organic electroluminescent diode is formed on the second passivation layer 64 and electrically contacts the driving drain electrode 54. The first electrode 66 is disposed in the pixel region as shown in FIG. 3.
In the related art shown in FIGS. 3-5, the driving active layer 58a has a wide channel width and a short channel length, so that the driving thin film transistor TD occupies a large amount of the pixel region. Therefore, an aperture ratio the bottom emission type organic electroluminescent display device is decreased. Further, since a large amount of current flows through the driving thin film transistor TD, current stress may be caused in the driving thin film transistor TD, thereby damaging the driving thin film transistor TD. Especially, when the DC bias is continuously applied to the driving thin film transistor TD, the electrical properties of the driving thin film transistors TD deteriorates and eventually malfunctions. Accordingly, the active matrix organic electroluminescent display device having the above-mentioned driving thin film transistor may show an residual image phenomenon, thereby causing bad display quality. Additionally, when the driving thin film transistor is deteriorated and malfunctioned by the electrical stress, a dot defect occurs in the pixel.